A common present day method of glass fiber production is the rotary or centrifugal process by which molten glass from a furnace is fed into a rotating refractory metal spinner having a large number of holes in its periphery and maintained at a relatively high temperature. Steam jets, air jets or gas burners are arranged around the spinner to provide downwardly directed gaseous streams of annular section. The spinner rotates at a relatively high speed and under the influence of centrifugal force the molten material is directed radially outwardly through the spinner holes and delivered into the peripheral gaseous blast which attenuates the material into fine diameter fibers.
A recent advancement in the rotary fiberization field provides for the manufacture of glass fibers having an average diameter below 7 microns without the necessity of using a relatively high temperature gaseous blast to attenuate the primary fibers. It was discovered that staple fibers having the desired diameter could be produced solely by passing molten material through the orifices of a rotor into a plurality of relatively cold and relatively low pressure gas flows, thus eliminating the hot gas blast used in the prior art and the fuel usage associated therewith.
Satisfactory production of fibers by the method described in the preceding paragraph requires that the glass compositions have certain defined characteristics. One of the primary requirements is that the glass have a relatively low melting and working temperature, so that it may be readily worked on a continuous and rapid basis with a minimum of energy required for melting it. The low working temperature is also needed to minimize corrosion and wear of the metal spinner. The glass must also have a Liquidus temperature that is sufficiently lower than the relatively low working temperature, thus enabling low temperature rotary fiberization by the above described method without devitrification, i.e., the undesirable formation of crystals. Additionally, the glass must have a viscosity at these relatively low working temperatures that is sufficiently low to permit high levels of fiber production. A final requirement is chemical durability. It is essential to use a glass composition that provides a fiber of extremely high weather resistance since the great amount of surface exposed per unit of weight renders the fibers sensitive to the corrosive influence of even such moisture as is present in the air.
Glass compositions heretofore used which meet the requirements of softening point, viscosity, Liquidus and durability, have also been characterized by relatively high working temperatures.